1. Field of the Invention
The present invention generally relates to the technical field of materials science, more specifically, it relates to the field of optical fiber, still more particularly to optical fibers produced in a microgravity environment and a method of production.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The following description of the art related to the present invention refers to a number of publications and other references. Discussion of such references herein is given to provide a more complete background of the principles related to the present invention and is not to be construed as an admission that such references are necessarily prior art for patentability determination purposes.
This invention described and claimed in the present application comprises the innovative combination of at least two technological areas: (1) the creation and management of microgravity environments; and (2) the construction and operation of optical draw towers. Combining those two areas as disclosed and claimed in this application results in the production of an optical fiber that has unique molecular structure and physical characteristics not attainable using conventional production methods.
A. What are Microgravity Environments?
Gravity is widely used to describe the attractive force between two objects with mass. Gravity is one of nature's fundamental physical forces and it affects all physical, chemical and biological processes on Earth. On the surface of the Earth, gravity applies an attractive force that attempts to accelerate an object towards the center of the Earth at approximately 9.8 meters/seconds2 (m/s2). Scientists believe that gravity is present throughout the universe, and that the strength of the gravitational force is directly proportional to the mass of an object and the distance from the object. For example, to reach a point where Earth's gravitational pull is reduced to one-millionth of that at the Earth's surface, you would need to be approximately 6.37 million kilometers from the Earth (that's over 16 times farther than the moon).
Since gravity pervades all aspects of the processes that take place on Earth, humans are accustomed to its impact. Until relatively recently, we have assumed that the only way for all processes to occur was with the known influence of gravity. The advent of space travel changed that paradigm. Space travel gave rise to experimentation in environments with significantly less gravity than perceived on Earth. That experimentation lead to the discovery that many processes and compositions of matter behave differently in so-called “microgravity environments.”
The term microgravity (μg) is generally defined as an environment where the local gravitational force is less than the standard gravity (g) experienced on the surface of the Earth (1 g). The exact force experienced in microgravity environments is a function of what method is used to create them. For example, a parabolic flight in an aircraft will result in a gravitational force that is about 1% of standard 1 g while a free flying satellite will experience about one millionth the force of standard 1 g. A potential impediment for understanding and furthering microgravity experimentation is that the ability to create and sustain a true reduced experimental gravity environment is much more complicated than merely reducing the effect of gravity.
As scientists have started to gain a better understanding of the impact of microgravity environments on processes, they have discovered that gravity's effect impedes a thorough understanding of many physical, chemical and biological phenomena. It is now widely known that gravity can mask some of the underlying, but critical, parameters that influence physical, chemical or biological processes. Gravity causes sedimentation, buoyancy and convection in liquids, creates hydrostatic pressure in liquids and modifies the behavior of liquid films on a surface. On the other hand, gravity tends to overwhelm capillary effects, multiphase flow, and diffusive transport processes.
As a result of the discoveries to date, it is generally understood that gravity is a factor in every process on Earth. In most processes, its impact is negligible. However in some processes, gravity is one of the key factors that leaves an imprint on the final structure of materials. Therefore the creation of reduced gravity environments for experiments has become a very important research area. While it is nearly impossible to create a gravity free environment, creating, monitoring and manipulating processes in a so-called microgravity environment has been achieved.
Microgravity experiments have revealed that even small forces present during a process can affect the process itself, i.e., the way the process takes place and the results. It is common to conduct experiments free of vibrational and other imparted forces. However, our present technological state does not allow full isolation from gravitational forces. It is well known that small forces acting on a process, whether vibrational or directional, can have similar effects on the process as gravity. For example, measurements on the International Space Station (ISS) showed large fluctuating accelerations, averaging approximately one thousandth of Earth's gravity, arose from movements of the crew, operation of equipment, spacecraft attitude adjustment, and waste dumps' operation. It was shown both theoretically and experimentally that those accelerations as well as other high-frequency vibrations can significantly impact the results of on-board experiments.
The similarity among the effects that different forces exert on processes, have brought about a pervasive misuse of terms. In many instances, reports of experimental results in a “microgravity environment,” are actually results obtained in environments free of outside forces and accelerations. That misuse of terminology brings about an important distinction. Production in an environment free of external forces, not merely production in microgravity, results in unique organic and inorganic materials. To stay consistent with the generally accepted terminology, when the term “microgravity” is used in this application, it refers to environments in which gravity is reduced and all of the other forces mentioned above have been reduced or eliminated as well.
B. How can Microgravity Environments be Achieved?
There are a number of ways to achieve microgravity environments as defined above. Each manner of creation is distinct from the others in terms of duration and quality of the microgravity environment, as well as the effort, including cost, required to achieve each one. These different methods are briefly discussed below.
1. Drop Towers.
A drop tower comprises a vertical shaft capable of providing a short-lived microgravity environment during the free fall of an experimental package for a time period proportional to the height of the tower. Microgravity levels between 10−3 g and 10−5 g have been measured in drop towers by adopting different techniques to counter the effect of acceleration due to gravity. Most experiments conducted using drop towers result in 2-3 seconds of microgravity. The Microgravity Center in Kami-Sunagwa, Hokkaido, Japan, however, is built in an old mine shaft and offers up to 10 seconds at 10−5 g. In addition to the short duration of microgravity, a major disadvantage of drop towers is the impact the package undergoes at the bottom of the tower.
2. Parabolic Flights.
Aircraft can fly in a series of parabolic arcs to achieve longer periods of microgravity, typically 20 to 25 seconds. The resulting microgravity environment during these parabolic arcs is 10−2 g to 10−3 g. In order to achieve microgravity, the airplane must climb rapidly until its nose is at about a 45-degree angle to the horizon then its engines are cut back. The airplane slows and the plane remains in free fall over the top of the parabola, and then nose-dives to complete the parabola. One of the major disadvantages of microgravity environment created using aircraft is the short duration.
3. Balloon Drop.
Stratospheric balloons can reach altitudes of 39 to 41 km and are being used regularly for a variety of missions, including space science missions. Such high altitude missions have also been used for microgravity research. Effectively, stratospheric balloons can be the equivalent of a very high drop tower. Stratospheric balloons can achieve a microgravity environment of 10−2 g to 10−3 g for up to 50 seconds. Stratospheric balloons present two significant drawbacks when used to create microgravity environments: (a) like the drop tower, the package must impact at the bottom of the drop, which can be mitigated to an extent by parachute; and (2) the overall microgravity environment is very poor due to random and irregular atmospheric buffeting and vibration.
4. Suborbital Rockets.
Suborbital rockets also fly parabolic paths but fly much higher than an airplane (up to 1500 km, depending on the rocket). This can result in up to 20 minutes of very high quality microgravity, 10−3 g to 10−4 g. The primary disadvantage of using suborbital rockets to conduct experiments in microgravity environments is the packaging requirements for the experiments. Such a package must fit within the volume constraints of the rocket and survive the high g launch load (up to 16 g) as well as the landing (up to 10 g shock).
5. Free Flying, Recoverable Satellites.
Free flying satellites (as contrasted with the International Space Station or the now retired space shuttles) provide an excellent microgravity environment for producing and processing materials. A free flying satellite in a 500 Km orbit can provide an environment of 10−5 g to 10−6 g for many months. The main drawbacks of using free flying satellites to conduct experiments or processes in microgravity environments are the requirements of safe reentry into the Earth's atmosphere and safe landing for recovery of the experimental package. That process is well understood and practiced routinely, however, it is more complex than a sounding rocket recovery system for example.
C. Summary of Available Microgravity Environments
Method of CreationDurationMicrogravityDrop tower2-10seconds10−2 g to 10−5 gParabolic aircraft25seconds10−2 g to 10−3 gBalloon drop30-50seconds10−2 g to 10−3 gSuborbital rocket4-20minutes10−3 g to 10−4 gFree flying satellitemonths10−5 g to 10−6 gD. Optical Draw Towers for Fiber Production
Optical fiber production is a useful process that can be greatly affected when conducted in a microgravity environment. The optical fiber production process begins with what is called a “preform” which is placed at the top of a drop tower. The preform usually comprises several layers of glass with different refractive indices which provide the core and cladding of the fiber. The preform can take on a variety of shapes, although the most common is circular. The size, shape and taper geometry of preforms are a function of the material to be used for the fiber as well as the specific requirements of the draw tower pulling system.
The method of producing optical fibers using a preform comprises the steps of: (1) placing the preform at the top of a draw tower (which is typically greater than 20 feet in height); (2) lowering the preform into a draw furnace; (3) using the draw furnace to heat the preform until a piece of molten glass, called a gob, begins to fall from the lower end of the preform due to the pull of gravity, much like hot taffy; (4) allowing the gob to pull a thin strand of glass as it falls, resulting in the first stage of the optical fiber; (5) cutting off the gob; (6) threading the resulting fine fiber strand into a computer controlled tractor assembly where it continues to be drawn; (7) using a laser system to monitor the fiber diameter; (8) using the laser system being to provide feedback to the computer controlled tractor assembly; (9) controlling the speed of the assembly so as to precisely control the diameter of the resulting fiber; (10) using a system of spray-on acrylic/polymer to coat the fiber, with the exact number and type of coatings applied being variable and depending on the specific application of the fiber; (11) illuminating the fiber using a UV lamp until the coating is hardened; and (12) winding the fiber on spools located at the bottom of the draw tower for shipment or further processing as necessary
Although the specific layout and operation will vary slightly from producer to producer, FIG. 1 illustrates the general process as well the typical components used in fiber optic production.
E. Fiber Production in Microgravity
It is well-known and well-understood that gravity affects the structure of a material lattice in two distinct ways: (1) directly through deformation that it imposes on the lattice during formation; and (2) indirectly through convection which induces mixing of the material as a function of the mass of each element. NASA, ESA and other space agencies have sponsored numerous experiments that have shown that production of materials in microgravity suppresses the creation of inclusions, dislocations, and crystallization centers.
During the late 1990's NASA and Canadian Space Agency researchers conducted experiments on KC-135 aircraft and suborbital rockets to try to determine whether ZBLAN optical fiber would crystallize while being drawn in a microgravity environment. They were able to demonstrate that when a small length of ZBLAN optical fiber, less than 1 meter in length, was drawn in microgravity it did not have crystallization. Budget issues and other priorities prevented the researchers from determining whether their results were anomalous or indicative of a new capability. That is a critical data point because manufacturers that produce ZBLAN optical fiber in a standard 1-gravity environment often end up with short segments (on the order of 1 meter) that are relatively crystal free while the overall optical fiber is heavily crystallized. Therefore, the fact that researchers were able to produce a short length of crystal free optical fiber in microgravity does not demonstrate that all fiber produced in microgravity would be free of crystallization.
Further, researchers have never been able to agree on the mechanism which would explain why ZBLAN produced in microgravity might be free of crystallization while ZBLAN produced in standard 1-gravity with the same process is heavily crystallized. Some NASA researchers believe that shear thinning in undercooled liquids increases the rate of nucleation and crystallization of glass forming melts. They believe that the shear of the melt can be reduced in low-g enhancing undercooling and glass formation.
Canadian researchers from the Department of Mechanical and Materials Engineering, Queen's University, Kingston, Ontario believe that the role of viscosity is critical in the devitrification mechanism of ZBLAN glass and in determining the optimum fiber-drawing temperature. Dennis S. Tucker, Space Optics Manufacturing Technology Center and Gary L. Workman and Guy A. Smith, Center for Automation and Robotics, University of Alabama in Huntsville carried out the experiment which pulled the small section of ZBLAN optical fiber during the microgravity portion of a KC-135 flight and they said, “It appears qualitatively, that gravity does have an effect on ZBLAN crystallization during fiber pulling. The exact mechanism is unknown at this time.” Although the initial research conducted during the late 1990's was useful, it was far from teaching or suggesting sufficient information to enable a person of ordinary skill in the art to conceive, let alone reduce to practice, the invention described and claimed herein without substantial, material, additional detailed research and experimentation.